1. Field of the Invention
This invention relates to the manufacture of a continuous sheet of flat glass by floating molten glass on a pool of molten metal while attenuating and cooling the glass. More particularly this invention relates to method and apparatus for manufacturing glass sheets over a wide range of thickness having improved optical quality and being of controlled width.
2. Description of the Prior Art
It has been proposed heretofore to form a continuous sheet of glass by depositing molten glass onto a bath of molten metal, such as molten tin or tin alloy, and drawing the glass along the bath of molten metal while cooling it and attenuating it to form a dimensionally stable ribbon or continuous sheet of glass which is then withdrawn from the bath for further processing. Early developments such as those of Heal, U.S. Pat. No. 710,357 and of Hitchcock, U.S. Pat. No. 789,911, disclose the manufacture of flat glass by continuously feeding molten glass onto a pool of molten metal to form a ribbon of glass which is cooled and drawn along the molten metal bath to form a finished ribbon of glass.
Glass produced according to these methods has been found to exhibit substantial optical distortion as reported by Pilkington in the file histories of the patents described below (Paper No. 5, pages 7 and 8 of U.S. Pat. No. 3,220,816). Optical distortion of a gross nature has thus been attributed in the art to a failure to break up the bottom surface of a discharged stream of glass. A failure to break up the bottom surface has the effect of maintaining imperfections earlier formed in conventionally refined and conditioned glass.
Nearly half a century following the disclosures of Heal and Hitchcock certain developments were made which permitted the commercial development of flat glass manufacture by a float process. These basic developments which have been made in a float process are the subjects of two patents to Pilkington, namely, U.S. Pat. No. 3,083,551 and U.S. Pat. No. 3,220,816. These patents disclose that molten glass, when discharged onto a pool or bath of molten metal, will spread laterally, if unhindered, to an equilibrium width and thickness and that a continuous ribbon of glass can be drawn from the molten glass which has spread out and is floating on the molten metal. These patents further disclose pouring molten glass onto molten metal in a manner such that the glass is allowed to fall freely onto the molten metal. The molten glass then separates into a rearwardly flowing stream and a forwardly flowing stream, both of which flow laterally. According to U.S. Pat. No. 3,220,816, the rearwardly flowing stream is comprised of glass which has been in contact with a refractory discharge member and has been contaminated by such contact, and this portion of glass spreads outwardly into the marginal portions of the finished ribbon and can be conveniently removed from the finished ribbon. These discoveries have permitted the formation of equilibrium thickness glass ribbons which have satisfactory surface quality and satisfactory chemical homogeneity for most present commercial uses.
However, as further developments have occurred, particularly those related to making thicker or thinner than equilibrium glass, workers in the art of glass manufacture have found that the commercially useful processes for making flat glass which depend upon the lateral unhindered flow of glass in its initial forming stages and which depend upon the free fall and rearward flow of at least a portion of the glass contribute to optical distortion in the finished glass which is unsatisfactory for uses which require glass of substantially higher optical quality than was required even a few years ago. For example, in the making of windshields from glass formed by the float process, it has been found desirable to employ glass which is relatively thin, that is, glass which is thinner than equilibrium glass and is of the order of 0.060 inch to 0.15 inch in thickness and preferably about 0.090 inch in thickness. Glass made by the float process to thicknesses on the order of less than about 0.15 inch is found to have greater apparent optical distortion than equilibrium thickness glass and great difficulty is encountered in making such thin glass with adequate optical quality to satisfy the requirements for automotive windshields.
The present invention is directed to a method and apparatus for manufacturing flat glass which has superior optical quality and which has further processing benefits which will be evident from the description which follows.